Over 60 million youth and adolescents participate in organized sports per year resulting in a high incidence of preventable, overuse injuries. A primary factor for overuse injury is that high amount of specialized participation and high intensity training by adolescent athletes without allowing for sufficient rest and recovery. The result is repetitive submaximal loading of the musculoskeletal tissue, leading to tissue failure. The objective of this NIAMS R21 proposal is to develop an innovative, non-invasive model that can detect the musculoskeletal tissue trauma and level of recovery that may be contributing to the overuse injuries associated with adolescent sports participation. Our approach utilizes two aims to 1.) demonstrate that diagnostic ultrasound obtained muscle cross-sectional area and echo intensity reflect physiological trauma and recovery from a bout of sport participation by comparing it to a common physiological gold standard (blood creatine kinase levels) and 2.) demonstrating that the model is feasible for use in a large cohort of adolescent participants in the field setting. Baseball pitching by adolescent baseball players will be the vehicle by which we will test our model given the high incidence of throwing-related overuse arm (shoulder and elbow) injuries we see in this population. Our rationale is for us to develop a non-invasive model that will be used in future R01 work to better understand overuse injury mechanisms for both the upper and lower extremity, in numerous sport and occupational activities. The application of the proposed model will be used to 1.) create, evaluate, and validate participation recommendations for those who participate in sports with high overuse injury risk, 2.) create evidence-based interventions (therapeutic modalities, pharmacology, and active rest models) for recovery following participation, and 3.) ultimately create evidenced-based strategies to prevent overuse injury in the youth and adolescent athlete.